1. Field of the Invention
This invention relates to visibility sensors. In particular, this invention relates to visibility sensors which are capable of measuring scattering amplitudes at different scattering angles. Multiple head redundancy ensures a higher degree of reliability, accuracy and self calibration.
2. Description of the Prior Art
In the most common type of application for a visibility sensor, the measurement parameter of interest is ultimately the extinction coefficient, which essentially gives the amount of attenuation that will be experienced by a light beam as it travels through an aerosol medium, such as atmospheric air. The extinction coefficient is related to the scattering coefficient through a complicated extinction integral which includes the scattering coefficient as a parameter. Visual range is the distance which a human observer will be able to see under certain background lighting conditions. Other factors, such as contrast, appear in the visibility range equation, but the major difficulty in determining visual range is in the visibility measurement. The visibility measurement is independent of the background lighting and is only a function of the total scattering within the atmosphere. An inexpensive and accurate means of reliably measuring this value is needed.
There are a wide range of prior art devices intended to measure the extinction coefficient of an aerosol. The forward scatter method offers several distinct advantages. In forward scatter meters the receiver accepts light from a source which has been scattered in a near-forward direction. The scattering coefficient in the direction of an angle of approximately 150 degrees between the axis of the beam and the axis of the receiver is nearly independent of the particle size distribution of the aerosol. For other angles, the ratio of the wavelength of optical energy used to probe the sample, to the size of the particles in the aerosol causes variations in the angular scattering coefficient for a given aerosol. Several regimes of size ratios are addressed in current theoretical literature.
Forward scattering meters are less sensitive to the particle size than backscatter meters or conventional linear path transmissometers. In addition, the total scattering coefficient is much greater in a general forward direction than a backward direction, and this manifests itself in an appreciably higher signal to noise ratio at the optical receiver. This translates, in turn, to a better sensitivity specification for forward scatter devices compared to other devices in the prior art.
U.S. Pat. No. 4,432,645 (Frungel, et al.) discloses a forward scatter visibility meter which includes a transmitter and a receiver which cooperatively couple at an obtuse forward scatter angle. The disclosure describes a calibration means which is used to measure the direct path light propagation characteristics of the aerosol media. This method will accountably modify the measurement process for dirty windows, aging light emitting elements and other factors which affect the overall through-path optical energy amplitude. By comparing the scattered amplitude with the direct path amplitude, a ratio is obtained which directly relates to the angular scattering coefficient at the angle indicated by the invention. It is designed to eliminate the effects of dust particles on transmitter and receiver windows on the measurement results. The transmitter beam direction is inclined downwards and the receiver direction of reception is inclined at an obtuse angle to the transmitter, also inclined downward; this reduces the noise level due to background scattered atmospheric optical energy. The invention includes an aperture with rain guiding ridges in order to keep raindrops from entering the active scatter volume which is comprised of the volume of aerosol within the intersection of the transmitter cone and the receiver cone. The calibration methodology disclosed requires manual intervention and an operator must occasionally service the unit in order to exploit the disclosed method.
U.S. Pat. No. 4,099,875 (McMahon, et al.) discloses an apparatus which determines the scattering coefficient of an aerosol media by measuring the amount of backscattered light. An optical transmitter beams optical energy into an aerosol medium and the backscattered light is measured in a time selective manner so that near and far field estimations may be made. A complicated processing phase is needed in order to turn the backscattered data into forward extinction coefficient data.
U.S. Pat. No. 4,329,054 (Bachalo) discloses a method of sizing aerosol particles by using laser light. A laser generator produces a pair of coherent laser beams of the same wavelength and intensity. These beams are directed and focused by focusing means along a first axis, and caused to cross at this first axis to establish an interference pattern. A collector for sensing the scattering caused by the particles in the interference pattern has a probe axis extending into the interference pattern. This probe axis is off-axis from the first axis. The visibility is determined from the information sensed by the collector. A sizing device for establishing the size of a particle, droplet or the like from the visibility is coupled to the visibility determining device and provides an output signal representative of the size. This apparatus uses a single frequency of optical energy.
U.S. Pat. No. 4,362,387 (Clark) discloses an invention for measuring the visibility of daylight by using polarization properties of sky and atmospheric light scattering. The intensities of two mutually perpendicular and linearly polarized components of the daylight are measured so that a polarization ratio can be calculated to provide a visibility index by which visual quality and visual range can be determined. Alternatively, the polarization properties at ultraviolet and infrared regions of the electromagnetic spectrum may be used in order to determine a three-dimensional topographical characterization of the sky. This apparatus is useful for remote types of observations, but is extremely expensive to facilitate and calibrate.
As seen above, prior art devices have attempted to characterize the scattering coefficients in various ways. Scattered light undergoes certain polarization phenomenon, and several prior art devices have attempted to exploit this phenomenon. These devices suffer the disadvantage of being extremely complex and expensive. The signal to noise ratios at the receivers to these devices suggest a very poor overall accuracy level. Linear line transmissometers suffer the disadvantage of being very difficult to calibrate. This is because they attempt to obtain a ratio between transmission in an unattenuating vacuum path, with that transmission obtained through the appropriate aerosol medium. There is essentially no adequate method of achieving this said vacuum path.
Furthermore, multiple frequency light sources, which study the variation on angular scattering coefficient with respect to optical energy wavelength suffer the disadvantage of being extremely complex since optical devices tend to operate at preferred, narrow-band frequencies. Complicated arrays of prisms and beam splitters are therefore required and the alignment and maintenance become prohibitive and unreliable.
Thus, prior art visibility sensors suffer the disadvantage of requiring special calibration means to measure the direct path light transmission properties and do not measure the angular scattering coefficient at several different angles. In sum, a visibility sensor with improved accuracy variability and reliability is desired without added expense or complexity.